Recent years have seen very rapid progress in the field of cryoelectronics. In particular, Josephson junction devices, with their high speed and low power dissipation, have come to show great promise as future digital devices.
In the Josephson devices developed up to now, however, the "0" and "1" logic values have been defined in terms of the voltage state across the junction. More specifically, the state in which the phase difference between the macroscopic wave-functions remains in circulation between the superconducting layers of the junction is defined as the logic value "0" or "1" while the other state, that is the zero-voltage state is defined as the other logic value. Therefore, although the power dissipation is indeed lower than that of semiconductor devices, it cannot be said that reduction of power dissipation has yet been pushed to the limit.
It is known that in a Josephson junction structure which is longer in one direction than the Josephson penetration depth a vortex current state will, under specific conditions, arise and persist, and, moreover, that it is possible to cause the vortex current distribution to propagate at high speed. The magnetic flux passing through the loop of the vortex current at this time is quantized and the quantized superconducting state behaves as an independent quantum. This vortex current and flux quantum are commonly referred to as a fluxon. A fluxon can be caused to propagate along a Josephson transmission line but since the voltage arising because of the propagation causes quasi-particle current, there is a gradual loss of energy which results in a decline in the propagation velocity. By applying an appropriate bias current perpendicularly to the junction, however, it is possible to supply energy to the fluxon, making it possible to increase the propagation velocity at will. Also, because of the Lorentz force arising between the magnetic flux and the bias current, it is possible to slow down the propagation velocity of the fluxon by applying as a damping current a bias current in the reverse direction from that mentioned above. Use of such fluxons as carriers of information will make it possible to realize a device with exceedingly low power dissipation.
A device employing fluxons within a Josephson junction structure has been disclosed in U.S. Pat. No. 3,676,718. In a device which employs fluxons as carriers of information it is intrinsically necessary to be able to start and stop propagation of the fluxons as desired. In order to make it possible to stop fluxons in an extended Josephson device, the invention of the aforesaid US patent provides portions on the Josephson structure where there is established a local minimum of the sum of the magnetic energy plus the Josephson coupling energy. In order to examine the performance and the inherent problems of the proposed device, it will be useful to take a look at the concrete structures which are used to provide stopping positions, i.e. positions at which the sum of the two energies is minimum. There are five such structures proposed in the specification of this patent:
A. Provision of regions where the Josephson critical current density is zero;
B. Varying the thickness of the Josephson junction oxide layer in the lengthwise direction of the extended Josephson device;
C. Provision of point sources of magnetic field at periodic points so as to enable minimization of energy;
D. Periodic application of currents to the extended Josephson device to enable minimization of energy; and
E. Providing variable self-inductance in the lengthwise direction of the extended Josephson device.
In the methods A, B and E, since the characteristic impedance of the extended Josephson structure encountered by the rapidly propagated fluxons differs between the stopping positions and the traveling regions, there is the disadvantage that the conditions under which the fluxons are caught by the stopping positions are too limited. Moreover, in the case of method B involving variation of the thickness of the oxide layer, there is the further disadvantage of the device fabrication process being made complex.
In methods C and D, it is necessary to minimize the energy by application of signals from outside (magnetic field in the case of method C and current in the case of method D), which disadvantageously complicates the device circuitry.
Further disadvantageously, in all of the methods of the patent it takes a considerable amount of time before a fluxon comes to complete rest at the position of minimum energy and this constitutes a bar to increasing the propagation clock rate of the fluxons to an extremely high level.
From the foregoing it can be seen that the prior art, including the above-mentioned U.S. patent, involves a number of intrinsic defects which prevent the realization of a practical extended Josephson device employing the behavior of fluxons, so that up to now there has been little promise of realizing high-performance devices capable of stable operation.